Many techniques for coating substances have been developed over the years. Nevertheless, there is a persistant need for new techniques able to coat a wide range of preformed solid particles. The reasons for this need are numerous. For instance, new applications for solid particulate materials and articles are developed consistently thereby requiring improved coating methods. Also, new coating materials are developed and require for their application new coating protocols. Furthermore, many existing coating protocols require the use of solvents which are often toxic or hazardous either to the environment, to personnel involved in a given particle coating protocol, or to the user of the particle. The latter situation arises when the final coated particle retains a finite amount of solvent(s) used in the coating of coated solid particles or articles. Further, solvents used in conventional solid particle or article coating processes may attack and partially dissolve the solid particles or articles being coated thereby altering properties of the solid particles or articles in some manner. Additionally, solvent-based coating formulations may cause mean size of particulate compositions being coated to increase measurably due to agglomeration caused by the nature of the solvent-based coating at the time of application.
A preponderance of current solid particle and article coating protocols involve deposition of liquid coating formulations on the surface(s) of the solid compositions being coated followed by solidification of said coating formulations. Solidification may occur because solvent is removed from the applied coating formulations, because the initially liquid coating formulations solidify due to cooling (e.g., they crystallize or pass below a glass transition temperature of the coating formulation during a cooling step), because initially liquid coating formulations are polymerized to a solid during the coating protocol, or because of a combination of these factors.
Supercritical fluids (SCF) in general, and supercritical carbon dioxide (SC CO.sub.2) in particular, are prime vehicles by which improved coating technology can be developed and applied. They are protrayed as environmentally friendly fluids which can be used for a variety of economically useful purposes.
U.S. Pat. No. 4,598,006 discloses a method for impregnating a thermoplastic polymer with an impregnation material such as a fragrance or pest control agent or pharmaceutical composition wherein the impregnation material is dissolved in a volatile swelling agent for the thermoplastic polymer, said swelling agent being maintained at or near supercritical conditions.
U.S. Pat. No. 5,043,280 discloses a method and apparatus for the manufacture of a product having a substance embedded in a carrier that involves the use of a supercritical gas or supercritical fluid (SCF). The method consists of passing a liquid that contains a substance and carrier through a nozzle into a chamber loaded with a SCF thereby forming a gaseous mixture of the SCF and liquid medium, followed by separating the gaseous mixture of SCF and liquid medium to produce a sterilized product comprising a substance embedded in a carrier.